adds gem 'wdm', '>= 0.1.0' if Gem.win_platform? to Gemfile

this is the recommended for windows from https://jekyllrb.com/docs/windows/

Gemfile

@@ -1,3 +1,4 @@
 source "https://rubygems.org"
 
-gem 'github-pages'
+gem 'github-pages'
+gem 'wdm', '>= 0.1.0' if Gem.win_platform?

Gemfile.lock

@@ -19,6 +19,7 @@ GEM
     faraday (0.12.2)
       multipart-post (>= 1.2, < 3)
     ffi (1.9.18)
+    ffi (1.9.18-x64-mingw32)
     forwardable-extended (2.6.0)
     gemoji (3.0.0)
     github-pages (150)
@@ -167,6 +168,8 @@ GEM
     net-dns (0.8.0)
     nokogiri (1.8.0)
       mini_portile2 (~> 2.2.0)
+    nokogiri (1.8.0-x64-mingw32)
+      mini_portile2 (~> 2.2.0)
     octokit (4.7.0)
       sawyer (~> 0.8.0, >= 0.5.3)
     pathutil (0.14.0)
@@ -193,12 +196,15 @@ GEM
     tzinfo (1.2.3)
       thread_safe (~> 0.1)
     unicode-display_width (1.3.0)
+    wdm (0.1.1)
 
 PLATFORMS
   ruby
+  x64-mingw32
 
 DEPENDENCIES
   github-pages
+  wdm (>= 0.1.0)
 
 BUNDLED WITH
-   1.14.3
+   1.16.0